Medical institutions are equipped with radiographic apparatus for acquiring a fluoroscopic image of a subject. Now, description will be given of a construction of a conventional radiographic apparatus. As shown in FIG. 12A, conventional radiographic apparatus 51 includes a top board 52 for supporting a subject M, a radiation source 53 for emitting radiation, and a radiation detector 54 for detecting radiation.
Radiation is emitted from the radiation source 53, and then radiation beams B transmit the subject M to enter into the detector 54. The detection signals outputted from the detector 54 are constructed into a fluoroscopic image.
Here, the radiation source 53 emits a uniform dose of radiation trough the radiation beams B. Accordingly, when an image is acquired under a state where the subject M is not placed on the top board 52, the pixel values are approximately uniform throughout the image. On the other hand, when a fluoroscopic image of the subject M is generated under the above state, a radiation fluoroscopic image P to be acquired has an insufficient dose of radiation at a center thereof. As a result, the image has a dark portion at the center of the subject M. See FIG. 12B. The reason therefor is as under. That is, the subject M has a largest thickness at a center thereof in a body axis direction. The thickness gradually decreases from the center toward the periphery. Accordingly, it is more difficult for the radiation beams B to transmit through the center of the subject M rather than the periphery.
The radiation source 53 is provided with a compensating filter 55 for avoiding such partial exposure variations to the detector 54. The radiation beams B transmitting through the compensating filter 55 have a high dose of radiation at a center c in the body axis direction than a periphery portion s. See U.S. Pat. No. 5,666,391.
The conventional configuration, however, has the following problem. That is, when radiation transmits the compensating filter 55, scattered radiation in modified traveling directions is generated, which leads to reduced visibility of the radioscopic image. The radiation emitted from the radiation source 53 transmits the compensating filter 55 toward the detector 54. Herein, radiation is to be generated having traveling directions modified due to collision with electrons that form the compensating filter 55. Such radiation is called scattered radiation (which may also be referred to as primary scattered radiation), and may lead to a lower-contrast radioscopy image. Especially, such problem will notably occur in cone-beam imaging where more scattered radiation is generated.
Such scattered radiation (which may also be referred to as secondary scattered radiation) is also generated when radiation beams B transmit the subject M. There is a conventional configuration of acquiring a radioscopic image as under. Specifically, an estimate calculation is performed on the assumption that radiation detected by the detector 54 is the sum of direct radiation (i.e., radiation that has reached the detector 54 without being scattered) and scattered radiation, whereby direct-radiation intensity is calculated. Based on this, a radioscopic image is to be acquired. A radioscopic image of excellent contrast may be acquired with only direct radiation.
Here, scattered radiation generated through the compensating filter 55 becomes obstructive in such estimation of direct-radiation intensity. In other words, the foregoing estimating method is performed on the assumption that scattered radiation is generated by the subject M. Accordingly, merely application of the conventional estimating method to the configuration where scattered radiation is generated at two parts of the compensating filter 55 and the subject M cannot achieve accurate estimation of direct radiation.
Then, methods of removing influence of scattered radiation to the compensating filter 55 include an approach of determining in advance direct radiation having transmitted the compensating filter 55. Specifically, in estimation of direct radiation, a slit S as in FIG. 13 is used for emitting only direct radiation toward each one detecting element e that forms the detector 54 to determine intensity of the direct radiation. According to this method, direct radiation to every detecting element e is successively determined while the slit S moves. This is very laborious. Moreover, the slit S need to move precisely. In addition, when the compensating filter 55 is replaced in accordance with imaging, the energy of X-rays emitted from the radiation source 53 is changed, or a distance varies between the radiation source 53 and the detector 54, scattering of radiation from the compensating filter 5 varies accordingly. As a result, direct radiation has to be determined with use of the slit S for every variation in imaging condition. Therefore, the method using the slit S is not considered as an actual solution.
In the conventional estimating method, radiation detected by the detector 54 may be separated into scattered radiation scattered through an object and direct radiation not scattered. Accordingly, a method may be considered that direct radiation is estimated in advance through execution of imaging under a state where the subject M is not placed on the top board 52 (i.e., air radiography). Specifically, direct radiation is estimated that transmits the compensating filter 55 in air radiography. Thereafter, radiography is again performed with the subject M placed on the top board 52. Accordingly, direct radiation may be estimated that transmits the subject M. On the other hand, this method needs two steps of calculations, i.e., determination of direct radiation transmitting the compensating filter 55 and determination of an amount of the radiation scattered through the subject M. Thus, time is involved until the end of the calculations. That is, estimate calculation of scattered radiation is performed in air radiography, and thereafter estimate calculation of scattered radiation is performed with the subject M being placed on the top board 52. Accordingly, time and labor will be spent the calculations.
This invention has been made regarding the state of the art noted above, and its object is to provide radiographic apparatus with a compensating filter that allows simple and accurate estimation of direct radiation to acquire a radioscopic image or a sectional image with excellent contrast.